Central offices are known to provide Centrex and/or advanced supplementary services. Many of these Centrex central offices are now being, or have been converted to support digital subscriber lines which provide an ISDN basic rate interface (BRI) capability.
The BRI is designed to provide two channels (2B) for providing channel service capable of transmitting either voice or data and one control channel (D) over a digital subscriber line (DSL). The BRI interface in present 5ESS.RTM. Centrex central office switches is arranged to provide for each terminal connected to the DSL a maximum of one B channel for voice service. That is, two voice channels to the same central office (CO) station terminal is not a supported service. Presently, however, some or many user station terminals exist which have two voice channels and, consequently, cannot operate effectively over the BRI interface that exists in present 5ESS Centrex central offices. Additionally, the user station terminals were required to be ISDN BRI compatible. Numerous existing PBX/KEY type telephone systems employ other than ISDN BRI compatible telephone terminal apparatus. A problem exists as to how to interface PBX/KEY type telephone systems to the BRI interface in the 5ESS Centrex central offices.